1. Field of the Invention
The present invention is directed to a surface resonator of the type suitable for use in nuclear magnetic resonance tomography, and in particular to a surface resonator in the form of a single loop of ribbon-like electrical conductor, wherein the capacitance between the ends of the conductor determines the resonant frequency.
2. Description of the Prior Art
It is known that hydrogen atom nuclei of an examination subject, i.e., protons bonded in water molecules, can be caused to precess from an equilibrium orientation or direction which is imposed on the nuclei by a fundamental magnetic field having a high static field strength. Precession of the nuclei is caused by radio-frequency excitation pulses. At the end of an excitation pulse, the atomic nuclei precess at a frequency dependent on the strength of the fundamental magnetic field and, after a predetermined relaxation time, return to the equilibrium direction as a consequence of their spin. An image can be produced by analysis of the signals generated by the precessing protons, the image being produced based on the spatial spin density or based on the distribution of the relaxation times within a body slice. The origin of the nuclear magnetic resonance signal arising from the precessional motion is identified by applying linear field gradients. These gradient fields are superimposed on the fundamental field, and are controlled such that only protons in a slice of the examination subject of interest are excited. The generation of images in this manner is generally referred to as nuclear magnetic resonance (NMR) tomography, or magnetic resonance imaging (MRI).
As is known, surface coils are used for obtaining images of certain body regions having a relatively small expanse, such surface coils generally being in the form of a flat coil with a plurality of turns. The surface coil is simply laid on the body part for which an image is to be generated, for example a vertebra, the middle ear, or on an eye. One embodiment of such a surface coil is referred to as a "loop-gap" resonator. This coil consists of a single turn of a ribbon-shaped metallic conductor having free tab or plate shaped ends which form a capacitor. The conductor is usually applied as a surface layer to a carrier of electrically insulating material, which may be a plastic such as polymethylacrylate. The capacitance can be varied by changing the spacing and size of the plate-shaped ends, as well as by selection of a dielectric. The capacitance can thus be brought into resonance with the inductance of the conductor, which may be a hollow cylinder. The energy can be inductively coupled into the coil via a coupling loop. Such a resonator is described in the Journal of Magnetic Resonance, Vol. 61, pages 571-578 (1985).
A microwave resonator in the form of a split-ring resonator wherein the conductor is ribbon-shaped is described in U.S. Pat. No. 4,446,429. This resonator, also in the form of a cylinder consisting of one turn or loop, is provided with at least one axial slot. The capacitance of the free ends disposed opposite each other again determines the resonant frequency.
The body region for which an image is generated using resonators of the type described above is determined by the circular cross-sectional area of the resonator. Because of the axial symmetry of such conventional resonators, the body region for which the image is generated is always of the same size, regardless whether the top or bottom surface of the resonator (defined by the edge of the conductor loop) faces toward the body surface. For obtaining images of regions which differ in size, different surface resonators having correspondingly different cross-sectional areas are needed.